As a plasma processing apparatus, there has been known an apparatus including a mounting table configured to mount thereon a target object (work piece) (for example, a semiconductor wafer, a glass substrate, and the like) in a processing chamber (see, for example, Patent Document 1). The plasma processing apparatus described in Patent Document 1 includes an electrostatic chuck configured to mount a wafer thereon. The electrostatic chuck has a central portion on which a wafer is mounted, and a flange portion formed to surround the central portion. Above the flange portion, an annular focus ring is provided to be apart from an upper surface of the flange portion. That is, there is a gap between a lower surface of the focus ring and the upper surface of the flange portion. Further, it is described that a heater is embedded in the focus ring, and a minute space for introducing a cooling gas is formed at the lower surface of the focus ring. The electrostatic chuck is connected to a RF power supply.
Patent Document 1: Specification of U.S. Pat. No. 6,795,292
In microprocessing for manufacturing a semiconductor device or a FPD (Flat Panel Display) using plasma, it is very important to control a temperature and a temperature distribution of a target object as well as a plasma density distribution on the target object. If the temperature of the target object is not appropriately controlled, uniformity of a surface reaction on the target object and uniformity of process characteristics cannot be secured. As a result, a production yield of semiconductor devices or display devices is reduced.
In order to control a temperature of a target object, there has been widely used a heater manner in which a heating element that generates heat when an electric current is applied is mounted on a mounting table and Joule heat generated from the heating element is controlled. Further, there has been widely used a method in which heat of a target object is absorbed by forming a coolant path within a mounting table. Furthermore, as described in Patent Document 1, in order to control a temperature of a focus ring, there has been provided a configuration in which a heater is embedded in a focus ring and a coolant for absorbing heat is introduced to a lower surface of the focus ring. By using these temperature control units, a set temperature of a target object and a set temperature of a focus ring are required to be maintained in an optimum temperature range for each processing condition of the target object.
The importance of the control of a focus ring temperature will be described below. FIG. 6A and FIG. 6B are graphs showing a dependency of a working shape of a wafer as a target object on a distance from a center of the wafer (wafer center). FIG. 6A is a graph showing a dependency of a hole depth on a distance from the wafer center. The horizontal axis represents a distance from the wafer center, and the longitudinal axis represents a hole depth. FIG. 6B is a graph showing a dependency of a hole shape on a distance from the wafer center. The horizontal axis represents a distance from the wafer center, and the longitudinal axis represents top CD. In both FIG. 6A and FIG. 6B, each temperature of a focus ring is plotted. As shown in FIG. 6A and FIG. 6B, a temperature of a focus ring greatly affects a hole depth particularly at a wafer end portion (for example, 145 mm to 147 mm). FIG. 7A and FIG. 7B are graphs showing a dependency of an etching rate (E/R) on a wafer position. FIG. 7A is a graph in a case where a temperature of a focus ring is not controlled. FIG. 7B is a graph in a case where a temperature of a focus ring is controlled to be low. In both FIG. 7A and FIG. 7B, X-axis and Y-axis are orthogonal to each other on the wafer, and results measured along these axes are plotted on the graphs. According to a comparison between FIG. 7A and FIG. 7B, in the case where a temperature of a focus ring is controlled to be low, an etching rate at the wafer end portion (for example, 145 mm to 147 mm) is closer to an etching rate at the wafer center. Thus, uniformity of an etching rate in the entire surface of the wafer is improved.
As such, in order to obtain uniformity of process accuracy in the entire surface of the target object, a temperature control unit for a focus ring as shown in the plasma processing apparatus described in Patent Document 1 as well as a temperature control unit for a target object needs to be provided. As a conventional mounting table, for example, a mounting table depicted in FIG. 8 may be considered. As depicted in FIG. 8, a mounting table 200 includes an aluminum base member 30 having therein coolant paths 200e and 200d, and an electrostatic chuck 60 having a wafer mounting surface 60d and a focus ring mounting surface 60e. The base member 30 serves as a high frequency electrode, and the electrostatic chuck 60 is made of ceramic or the like and provided on the aluminum base member 30. The electrostatic chuck 60 has a central portion 60g and a flange portion 60h formed to surround the central portion 60g. An upper surface of the central portion 60g serves as the wafer mounting surface 60d, and an upper surface of the flange portion 60h serves as the focus ring mounting surface 60e. Heaters 60c and 7c are provided under the wafer mounting surface 60d and the focus ring mounting surface 60e, for example, within the electrostatic chuck 60, and configured to independently control temperatures of the wafer mounting surface 60d and the focus ring mounting surface 60e, respectively. In order to control temperatures of the wafer and the focus ring to be increased, the heaters 60c and 7c supply heat to the wafer mounting surface 60d and the focus ring mounting surface 60e, respectively. Further, in order to control temperatures of the wafer and the focus ring to be decreased, heat is transferred and absorbed from the wafer mounting surface 60d and the focus ring mounting surface 60e to the coolant paths 200e and 200d within the aluminum base member 30, i.e. in a vertical direction, respectively.
A temperature control of the mounting table configured as described above is verified, and a result of the verification is shown in FIG. 9. FIG. 9 is a graph showing temperatures measured at each distance (radius) from the center of the mounting table, and the horizontal axis represents a radius, and the longitudinal axis represents a temperature. In FIG. 9, a radius on the inner side of a radius indicated by a dotted line corresponds to a wafer region on which the wafer is mounted, and a radius on the outer side of the radius indicated by the dotted line corresponds to a FR region on which the focus ring is mounted. In FIG. 9, a heater in the wafer region is turned off, and a heater in the FR region is turned on. That is, there is provided a measurement result in a case where a wafer temperature is not controlled, but only a focus ring temperature is controlled. As shown in FIG. 9, thermal interference occurs in the vicinity of the radius indicated by the dotted line, i.e. between the wafer region and the FR region, and a temperature at a wafer end portion is increased. That is, when the wafer mounting surface and the focus ring mounting surface are formed on the electrostatic chuck, there occurs thermal diffusion from a focus ring side to a wafer side. Heat introduced into the electrostatic chuck flows not only in the vertical direction starting from the electrostatic chuck to the coolant paths in the aluminum base member but also in a horizontal direction (diametrical direction of the mounting table) within the electrostatic chuck and at a portion above the coolant paths within the aluminum base member.
For this reason, as described in Patent Document 1, by forming a gap between the lower surface of the focus ring and the upper surface of the flange portion of the electrostatic chuck, the focus ring is not in direct contact with the electrostatic chuck. With this configuration, the direct thermal diffusion from the focus ring side to the electrostatic chuck side may be suppressed. However, if members supporting the focus ring and the electrostatic chuck are thermally connected to each other, indirect thermal interference may occur unless a height position of a coolant path is considered. For this reason, there has been demanded a unit capable of independently controlling a temperature of a target object and a temperature of a focus ring.
Further, even if a temperature of the wafer mounting surface and a temperature of the focus ring mounting surface are controlled separately, there are problems to be solved as follows. Recently, a set temperature of the focus ring has been demanded to be higher than a set temperature of the target object. By way of example, there has been demanded to generate a temperature difference of about 100° C. or more. However, if a temperature of the heater right under the wafer mounting surface is controlled to have a temperature difference of about 40° C. or more from a temperature of the heater right under the focus ring mounting surface, the electrostatic chuck made of ceramic may be damaged due to the thermal expansion. FIG. 10 shows a result of verification of a maximum stress generated at the electrostatic chuck due to a temperature difference. The horizontal axis represents a temperature difference, and the longitudinal axis represents a maximum stress generated at a measurement position. The square legend (temperature difference: 40° C., maximum stress: 388 MPa) shows the case where a temperature of the focus ring is controlled to be lower than a temperature of the target object, and the other legend shows the case where a temperature of the focus ring is controlled to be higher than a temperature of the target object. A reference value of a maximum stress at which damage occurs is 190 MPa. As shown in FIG. 10, when a temperature difference is 40° C. or more, a maximum stress exceeds the reference value of 190 MPa. Such damage is likely to occur at a position where a thickness of the electrostatic chuck is changed. By way of example, as depicted in FIG. 8, such damage is likely to occur at a step-shaped portion at the boundary between the wafer mounting surface 60d and the focus ring mounting surface 60e. Further, since a heater cannot be arranged at a portion where the electrostatic chuck is screwed to the aluminum base member, such a temperature difference can be easily generated. As a result, damage is likely to occur at such a portion. By way of example, as depicted in FIG. 8, when the aluminum base member 30 is connected to a supporting member 40 with a screw 8e, the aluminum base member 30 is screwed with the supporting member 40 by inserting the screw 8e into a through hole 60i formed within the flange portion 60h of the electrostatic chuck 60, a through hole 30a (whose inner surface may be screw-cut) formed within the aluminum base member 30, and an insertion through hole 40a formed within the supporting member 40 and having a screw-cut inner surface. In this case, since the through hole 60i is formed within the electrostatic chuck 60, the heater 7c cannot be arranged therein. Therefore, a temperature difference can be easily generated at a portion where the through hole 60i is formed. As a result, damage is likely to occur at this portion.
Further, in the mounting table as described in Patent Document 1, a member constituting the wafer mounting surface is different from a member constituting the focus ring mounting surface. Therefore, it is possible to reduce an effect of thermal stress deformation caused by the thermal expansion difference. By way of example, it is proposed to separately prepare an inner ceramic plate configured to mount a wafer and an outer ceramic plate configured to annularly surround the inner ceramic plate. Further, heating is controlled with a heater embedded in each of the ceramic plates. Furthermore, an aluminum plate including a coolant path is provided in each of lower layers of the inner and outer ceramic plates, and a heat flux is controlled in the vertical direction.
However, in the above-described configuration, a RF power needs to be applied to each of the separately provided aluminum plates. Otherwise, it is necessary to apply a power to each of the aluminum plates through divided power supply lines, each having a matcher, from a single RF power supply. In order to perform a complicated application sequence such as simultaneous application of a RF power in a pulse waveform, or the like, a configuration of an apparatus becomes complicated. Therefore, it is desirable to apply a power from a single RF power supply. Further, since a wafer mounting member and a focus ring mounting member are different in an area and a thickness, the wafer mounting member and the focus ring mounting member have conductance components greatly different from each other. By way of example, if each of the wafer mounting member and the focus ring mounting member includes a ceramic plate, a conductance component greatly varies depending on an area and a thickness of the corresponding ceramic plate. For this reason, if a power supply line is divided into a power supply line connected to the wafer and a power supply line connected to the focus ring, a RF power may not be distributed appropriately. As a result, a sheath field generated at a plasma interface may be non-uniform on the surface of the wafer and the surface of the focus ring. In such case, there is a problem that a desired semiconductor device cannot be manufactured.
As such, in the present technical field, there have been demanded a mounting table and a plasma processing apparatus capable of independently controlling the temperature of the target object and the temperature of the focus ring, generating a great temperature difference between the target object and the focus ring, which is limited by the thermal stress deformation, and generating a uniform sheath field on the surface of the wafer and the surface of the focus ring in a simple configuration.